The present invention is related to positive radiation sensitive resists used in electron and x-ray lithography.
Fine-line lithography is used to transfer patterns into a radiation-sensitive resist in many areas of micro-fabrication, e.g., materials science, optics and electronics. A radiation-sensitive resist is one in which chemical or physical changes induced by ionizing radiation allow the resist to be patterned. Most of the resists used in fine-line lithography are polymers functionally classified as belonging to two groups depending on whether their solubilities in the appropriate developers are markedly enhanced or diminished by irradiation. These resists are commonly called positive and negative resists, respectively.
A molecule of an ideal polymer resist consists basically of a chain of monomer units polymerized into a backbone composed primarily of carbon atoms. All polymer chains will not necessarily be of the same length. To characterize the distribution of polymer chain lengths, it is common to compare two different moments or averages of the distribution. For the purposes herein, the ratio of the weight average M.sub.w, to number average, M.sub.n, molecular weight will be used to characterize the distribution of polymer chain lengths. The number average molecular weight is defined as EQU M.sub.n =.SIGMA..sub.i N.sub.i M.sub.i /.SIGMA.N.sub.i
where N.sub.i is the number of moles of polymer having molecular weight M.sub.i. The weight average molecular weight is defined as EQU M.sub.w =.SIGMA..sub.i N.sub.i M.sub.i.sup.2 /.SIGMA.N.sub.i M.sub.i
where N.sub.i is the number of moles of polymer having molecular weight M.sub.i. If all polymer chains are of equal length then M.sub.w /M.sub.n =1.
To use a polymer resist for lithographic patterning of a substrate, the resist must be first coated on the surface of the substrate. The resist is dissolved in a suitable solvent, and the solution is applied to the substrate by some method such as dipping or spinning. As the solvent evaporates, the resist passes from a solution to a dense, amorphous mat of tangled polymer chains. The physical and chemical properties of the resultant glassy material depend both on the characteristics of the individual polymer molecules making up the mat and on the organization of these individual polymer molecules in the mat. The organization of these individual polymer chains in the mat may be altered by baking the resist at elevated temperature. For most polymer resists there is a temperature above which the chains re-organize into a more ordered state. This is referred to as the glass transition temperature, Tg.
When a polymer resist is subjected to ionizing radiation, atomic bonds are ruptured resulting in two types of molecular rearrangements. In chain scission events, a carbon back-bone bond is broken resulting in two shorter chains, each comprising a fraction of the molecular weight of the original molecule. In cross-linking events new bonds are formed between atoms in the side groups to other side group atoms in the same or neighboring molecules. These additional bonds tie the polymer molecules together into molecules of higher molecular weight.
For both positive and negative polymer resists, the solubility of the resist in the developer increases with decreasing molecular weight. For positive resists the decrease in the molecular weight due to irradiation renders the exposed resist readily soluble in a developer in which the unexposed resist has only a very small solubility. For negative resists the converse is true.
It is common to characterize resist material by sensitivity and contrast. The sensitivity of a resist, is defined as the incident dose required to cause a change in the soluability of the resist in developer sufficient for production of a lithographically useful image. For the purposes herein, a change in the solubility of resist in developer of 1,000 .ANG./min will be used to define the sensitivity. The sensitivity of a resist is influenced by several parameters. These include energy of the ionizing radiation, resist thickness, substrate material, polymer molecular weight, distribution of molecular weights and activity of the developer. Contrast, .gamma., is defined as the slope of the linear portion of the curve of remaining resist thickness versus the natural log of dose for fixed developer condition. Mathematically .gamma.=log (Q.sup.0 /Q.sup.1)-1 where Q.sup.0 is the linearly extrapolated dose for full thickness and Q.sup.1 is the linearly extrapolated dose for zero remaining resist thickness after development. High contrast is desirable for obtaining better edge definition and finer lines. It should be noted that the contrast can be calculated directly from a plot of etch rate vs. dose in the exposure by computing thickness of resist remaining after development.
One of the most used positive polymer resists is poly(methylmethacrylate), PMMA. It was one of the first radiation-sensitive resist materials to have been used for fabricating electronic components. It is attractive because of a number of its properties, i.e., insensitivity to light, high resolution, ease of availability.
The properties of PMMA as a resist for electron beam lithography have been summarized by several authors, see for example, S. Greenich in Electron Beam Technology in Microelectronic Fabrication ed. by G. Brewer, Academic Press, New York, 1980, or N. D. Wittels in Fine Line Lithography ed. R. Newman, Academic Press, New York, 1980. Use of PMMA as a radiation sensitive resist is described by I. Haller and M. Hatzakis, U.S. Pat. No. 3,535,137 and developer characteristics of the resist are mentioned by C. A. Cortellino, U.S. Pat. No. 4,078,098. It should be noted that improvement of sensitivity and contrast by simultaneously controlling the resist molecular weight distribution, molecular weight and baking temperature is not anticipated in this prior art.